1. Field of the Invention
The invention pertains to the field of gas flow control. More particularly, the invention pertains to control of gas flow in drying hoppers.
2. Description of Related Art
Especially when heating and drying granulates in drying hoppers where the granulate is continually taken out at the bottom with a desired temperature and the granulate to be dried is added batch wise at the top of the hopper, it is very difficult to reach an optimal gas throughput. Therefore it is common to choose a gas flow much too high just to be sure that all of the granulate gets really heated to the desired final temperature. This results in great losses of energy.
If the gas flow is unnecessarily large additional losses of energy are encountered when the return gas coming from the drying hopper flows through a drier for the elimination of its moisture content and from there after being reheated to the drying temperature in a closed circuit back to the bottom of the drying hopper. This is because dryers working on the principle of cooling or adsorption can deliver a low dew point of the dried air in an economical way only whenever the return air to be dried has a low enough temperature. In order to guarantee such a low temperature of the return air especially under changing drying conditions special cooling aggregates are installed in the ducting of the return air. This results in high installation costs and a continuous loss of energy through the cooling of the return air and its following reheating.
A similar process to the present invention was described in the present inventor's 1981 German patent application 3131471 and corresponding U.S. Pat. No. 4,413,426, issued in 1983 and entitled “Method and apparatus for drying moist exhaust air from one or more bulk material drying hoppers”. In these patents the amount of airflow is reduced when the return air temperature increases, and reduced with sinking return air temperature. In accordance with claim 5 of the German patent this control shall ensure that the return temperature remains always above the room temperature.
Sadly it appeared that this process does not function well or not at all under a number of drying conditions. This happens when the granulate to be dried has a temperature which is above room temperature whenever the granulate is added at the top of the drying hopper. This happens easily when the room temperature falls during the drying operation or when warm granulate shall be dried.
Under these conditions following the process of U.S. Pat. No. 4,413,426, the amount of airflow would be reduced and therefore would not be any more sufficient to dry the granulate. At the same time it can happen that the granulate to be dried has a temperature quite a bit lower than room temperature for instance when granulate is taken out during the winter from an outdoor silo. In such a case the gas flow would be unnecessarily high. The drying would take place successfully but it would be performed with an unnecessarily high energy consumption.
Adsorption dryers like those described in the present inventor's 1997 U.S. Pat. No. 5,659,974, “Method for regeneration of an adsorbent material containing moisture and apparatus therefor” or U.S. Pat. No. 5,688,305 “Method and Device for drying of moist gases”, consist of one or more chambers filled with adsorption material which cyclically dry the return air stream and then are regenerated by an secondary air stream, mostly highly heated room air. As described in FIG. 3, the adsorption material can be arranged on a rotor or a honeycomb material, which cyclical is moved through a dry air adsorption and a regeneration zone.